The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Low pressure plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density, in the range of 1011-1013ions/cm3.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to xe2x80x9cdewetxe2x80x9d from the sidewalls of the opening. Because the metal is typically deposited at relatively high temperatures as a result of the sputtering process, the deposited metal ions and atoms can have a relatively high degree of mobility which enables the deposited metal to form beads on the interior surfaces of the opening rather than a smooth continuous layer as desired. This tendency to form beads is often referred to as xe2x80x9cdewettingxe2x80x9d and can cause voids to form in the deposited metal.
To prevent or reduce dewetting, it has been proposed to deposit the contact metal in two separate layers. First a relatively thin xe2x80x9cwetting layerxe2x80x9d is deposited to coat the interior surfaces of the opening. This wetting layer is often deposited at relatively low temperatures by controlling the substrate temperature to reduce the metal ion and atom mobility. Consequently, the crystalline bonds of the deposited metal can hold the metal atoms and ions in place as the wetting layer is deposited to reduce the occurrence of dewetting. Typical wetting layer metals for the deposition of aluminum alloy contacts include AlCu and AlSiCu alloys.
Once the wetting underlayer has been deposited and cooled to lock the wetting layer in place, the remaining metal contact layer is deposited and planarized, typically in a separate sputtering deposition or chamber. The presence of the initial wetting layer prior to the deposition of the remaining contact metal or metals has been found to significantly reduce dewetting of the metal being deposited onto the dewetting underlayer. As a consequence, the formation of voids can be reduced.
However, when the wetting underlayer is deposited into high aspect ratio openings in an inductively coupled plasma sputtering chamber, it has been found that the wetting layer itself has a tendency to dewet, such that the wetting layer may not form a continuous layer over the interior surfaces of the opening. This tendency to dewet is particularly frequent on the sidewall surfaces of the opening which typically receive a relatively thin coating of material as compared to the bottom surfaces of the opening in inductively coupled plasma sputtering processes. On those surfaces of the opening in which the wetting layer does dewet, the metal later deposited on those dewetted surfaces also tends to dewet, thereby increasing the chances of the formation of an undesirable void in the metal contact layer.
It is an object of the present invention to provide an improved method and apparatus for sputter depositing a layer which enhances both sidewall and bottom coverage.
These and other objects and advantages are achieved by a plasma generating apparatus in which, in accordance with one aspect of the invention, a layer of relatively pure aluminum is sputtered in an inductively coupled plasma chamber to form a wetting layer prior to deposition of the remaining metal contact layer. It has been found that sputtering relatively pure aluminum (preferably 99.999% pure) to form a wetting underlayer can significantly reduce the incidence of dewetting of the wetting layer. As a consequence, coverage of sidewalls of channels, vias and other high aspect ratio openings and structures having a sidewall in a substrate may be improved so as to substantially reduce the formation of voids in the overlayer.
In an alternative embodiment, the wetting underlayer may be sputtered in a hydrogen-argon mixture. The hydrogen combines with the sputtered aluminum to form a wetting layer which includes molecules of aluminum hydride. It is believed that these aluminum hydride molecules have a relatively low sticking coefficient on the wetting layer surface. As a result, it is also believed that the aluminum hydride molecules will facilitate formation of a continuous, conformal wetting underlayer with good sidewall coverage.